Reduce "String.Concat(a, b)" to "a + b"

Decompiler.csproj

@@ -66,6 +66,7 @@
     <Compile Include="src\ByteCode.StackAnalysis.cs" />
     <Compile Include="src\StackExpression.cs" />
     <Compile Include="src\StackExpressionCollection.cs" />
+    <Compile Include="src\Transforms\Ast\Idioms.cs" />
     <Compile Include="src\Transforms\Ast\RemoveDeadLabels.cs" />
     <Compile Include="src\Transforms\Ast\RemoveEmptyElseBody.cs" />
     <Compile Include="src\Transforms\Ast\RemoveGotos.cs" />

src/AstBuilder.cs

@@ -27,6 +27,7 @@ namespace Decompiler
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.RemoveGotos(), null);
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.RemoveDeadLabels(), null);
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.SimplifyTypeReferences(), null);
+			astCompileUnit.AcceptVisitor(new Transforms.Ast.Idioms(), null);
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.RemoveEmptyElseBody(), null);
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.RestoreLoop(), null);
 			astCompileUnit.AcceptVisitor(new Transforms.Ast.RemoveDeadLabels(), null);

src/Transforms/Ast/Idioms.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Generic;
+using Ast = ICSharpCode.NRefactory.Ast;
+using ICSharpCode.NRefactory.Ast;
+using ICSharpCode.NRefactory.Visitors;
+
+namespace Decompiler.Transforms.Ast
+{
+	public class Idioms: AbstractAstTransformer
+	{
+		public override object VisitInvocationExpression(InvocationExpression invocationExpression, object data)
+		{
+			base.VisitInvocationExpression(invocationExpression, data);
+			
+			// Reduce "String.Concat(a, b)" to "a + b"
+			MemberReferenceExpression target = invocationExpression.TargetObject as MemberReferenceExpression;
+			if (target != null &&
+			    target.MemberName == "Concat" &&
+			    invocationExpression.Arguments.Count == 2 &&
+			    target.TargetObject is IdentifierExpression &&
+			    (target.TargetObject as IdentifierExpression).Identifier == "String")
+			{
+				ReplaceCurrentNode(
+					new BinaryOperatorExpression(
+						invocationExpression.Arguments[0],
+						BinaryOperatorType.Add,
+						invocationExpression.Arguments[1]
+					)
+				);
+			}
+			
+			return null;
+		}
+	}
+}